Numerous genes conferring resistance to pathogens have been identified and used in plant breeding. However, single-gene pathogen resistance in plants often becomes ineffective due to the emergence of new virulent races of the disease agent. In contrast, durable disease resistance in plants is generally thought to be controlled by multiple genes.
The wheat (Triticum aestivum) quantitative trait locus, Lr34, provides durable adult plant resistance to the biotrophic fungi causing the diseases leaf rust, stripe rust, stem rust and powdery mildew (Dyck, 1977 and 1987; German and Kolmer, 1992; Bossolini et al. 2006; Spielmeyer et al. 2008). This is despite the limitation that it is not effective at the seedling stage under normal field conditions. Cultivars with the resistance locus Lr34 such as Frontana have had effective durable resistance to the leaf rust fungus Puccinia triticina Eriks (Dyck et al., 1966; Singh and Rajaram, 1994). To date, isolates of P. triticina with complete virulence to Lr34 have not been detected (Kolmer et al., 2003).
Lr34 resistance has remained genetically inseparable from Yr18 that confers resistance to stripe rust (P. striiformis) (Singh, 1992a; McIntosh, 1992). Co-segregation of Lr34/Yr18 with other traits such as leaf tip necrosis (Ltn1), powdery mildew (recently designated Pm38), tolerance to barley yellow dwarf virus (Bdv1) and spot blotch (Bipolaris sorokiniana) have been documented (Singh, 1992a,b; McIntosh, 1992; Joshi et al., 2004; Spielmeyer et al., 2005; Liang et al., 2006). These multi-pathogen resistance traits have made the Lr34/Yr18 locus one of the most valuable gene regions for disease resistance breeding in wheat.
A few rust resistance genes have been isolated and cloned from wheat (Feuillet et al., 2003; Huang et al., 2003; Cloutier et al., 2007) and other cereals (Collins et al., 1999; Brueggeman et al., 2002) and are predominantly from the nucleotide binding site-leucine rich repeat (NB-LRR) class of major resistance (R) genes. The only known exception is the barley Rpg1 rust resistance gene which encodes a protein kinase. These genes encode gene-for-gene resistance against single pathogens and generally lead to hypersensitive responses in the plant tissues upon infection. In contrast, Lr34 confers a broad spectrum resistance against several obligate biotrophic pathogens including fungi from the Ascomycetes and Basidiomycetes. Rubiales and Niks (1995) reported that Lr34 is associated with reduced intercellular hyphal growth but not with a hypersensitive response or papilla formation.
The molecular basis of quantitative non-race-specific, adult plant pathogen resistance-type or partial resistance encoded by genetic systems such as, for example, Lr34 therefore remains unknown.